Scientists trying to piece together the story of how the universe evolved from clumps of dark matter into the sparkling galaxies of today have had to work around a central problem -- no reliable technique to determine the age of small stars like the sun, which are the most common.
An astronomer at the Space Telescope Science Institute has taken a big step in resolving the puzzle. Jason Kalirai used fresh stellar corpses of sun-like stars to serve as a key to time.
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“It’s like putting hands on a clock,” Timothy Beers, director of Kitt Peak National Observatory, told Discovery News.
By analyzing four of these newly dead stars, Kalirai determined that the inner halo of stars surrounding the Milky Way is 11.4 billion years old.
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The halo is spherical cloud of objects that are not orderly rotating around the center of the galaxy, like the sun and other stars in the Milky Way’s disk do.
“You’re either rotating around the center in an organized fashion -- that’s the disk -- or you’re supported by rapid, random motions. That’s the halo,” Beers said.
Within the halo are at least two populations of stars, again characterized less by location and more by relative motion.
How the stars came to be part of the Milky Way remains a mystery, but Kalirai’s technique can be used to home in on when they arrived.
“When an object comes into the halo, it gets shredded from its parent galaxy and depending on what the orbit was of that parent galaxy it is going to be moving in that orientation. By knowing when those stars formed, we have a limit on when they were accreted into the Milky Way because they couldn’t have been accreted before they were formed,“ Kalirai told Discovery News.
“Until this came along there was really no good way to quantify the age of the stellar distribution,” Beers added.
“Jason takes a handful of a set of stars, believed to be part of the inner halo population, age them and assigns them to a population from which they were drawn,” said Beers, who called the approach “ingenious.”
Kalirai chose so-called white dwarf stars as his portholes because they are simple stars whose light can be broken down into composite wavelengths impregnated with features directly relating to the star’s properties, such as its mass and temperature.
“You can measure it in a straightforward way. You can’t do that for normal hydrogen-burning stars,” Kalirai said.
White dwarfs are what remain after sun-like stars burn through all their hydrogen and shed their outer layers. They are 1 million times more dense than anything on Earth.
“If you took a tablespoon of material from the surface of a white dwarf, it would weigh as much as a school bus here on Earth,” Kalirai said.
They are also terribly common in the Milky Way. About 98 percent of the stars in the galaxy will end their lives as white dwarfs.
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Kalirai picked the newest corpses, ones uncomplicated by cooling, to resurrect key details of the stars they once were, such as their mass and temperature, then used that information to determine their age.
The next step is to find similar stars in the outer halo population, which is believed to contain objects that predate the Milky Way itself. Astronomers suspect some so-called globular clusters -- massive, densely packed clusters of stars -- are remnants of smaller proto-galaxies axed by gravity and reassembled into building blocks for the Milky Way.
“Galaxies don’t just collapse as one big ball. They get assembled from many smaller pieces that become larger as they interact with each other,” Beers said.
“How and when they do it is exactly the story we want to tell, “ he added.
“This technique enables us to make meaningful comparisons between the stellar populations of the halo. One can then work out a timeline for the assembly of the galaxy,” Beers said.
Kalirai’s research appears today on Nature's website.